The invention relates to radio frequency modulators, and more particularly to modulators accommodating both digital and analog modulating signals, and most particularly to modulators which limit the bandwidth of transmitted signals.
There are many situations in which it is desirable to limit the bandwidth of a modulated carrier signal. In digital communications, for example, the square waveforms used to represent binary bits generate signal components far removed in frequency from the carrier signal. Not only is this wasteful, since such components are unnecessary to recover the digital information upon reception, but it is also likely to cause interference with signals on adjacent frequencies through signal "splatter". The bandwidth can also be limited for voice transmissions since a bandwidth of only 6 KHz is required for full intelligibility for AM signals. Other modes such as single sideband require less than half this bandwidth.
It is well known to use analog multi-pole filters to limit the bandwidth of such signals. However, such filters are complex and are subject variably from radio to radio due to differences in component values and from time to time due to environmental conditions. They are also fixed in frequency and bandwidth and thus are not suitable for radio transmitters which must accommodate a wide variety of data transmission rates, including both digital and analog information, which are possible with modern digitally synthesized exciters. Typically, a digitally synthesized exciter may be required to transmit signals at data rates in the range of 75 bps to 160,000 bps. If television data were to be transmitted, data rates of up to 4,000,000 bps would be encountered.
It is therefore an object of the present invention to provide an FM modulator which will accommodate a wide range of frequencies and transmission data rates.
It is another object of the present invention to provide an FM modulator which will provide digital premodulation bandpass filtration.
It is a further object of the present invention to provide an FM modulator which is compatible with digital synthesizers.
It is yet another object of the present invention to provide an FM modulator which is compatible with both digital and analog modulating signals.
It is yet a further object of the present invention to provide an FM modulator which is less complex than previous modulators using analog multi-pole bandpass premodulation filters.
It is still a further object of the present invention to provide an FM modulator which is less expensive than previous modulators using multi-pole bandpass premodulation filters.